The measurement of morphological characteristics of solids, such as catalysts, catalyst supports, pigments, clays, minerals, pharmaceutics, and composite materials is an important aspect of analytical chemistry and quality control for manufacturing of numerous products.
For example, a very useful morphological characteristic of a solid is its surface area. One of the most widely used techniques for surface area determination is that of gas sorption. Gas sorption techniques utilize a theoretical model wherein the surface of a solid, the adsorbent, is characterized as being covered by a monolayer of closely packed molecules of an adsorbed gas. After adsorption on to the adsorbent, the condensed, relatively non-mobile gas phase is referred to as the adsorbate; whereas, the highly mobile gaseous phase is referred to as the adsorptive. If one can determine the amount, usually expressed in millimoles, of adsorbate which forms the monolayer, the area which is covered by the monolayer can be calculated from the product of the number of molecules in the monolayer and the cross sectional area of each molecule. In 1938 Branauer, Emmett, and Teller (J. Am, Chem. Soc. Vol. 60, 2309) described a mathematical equation, referred to as the BET equation, for determining the amount of adsorbate in the monolayer from the adsorption isotherm of the adsorbate.
The adsorption isotherm is a plot of the amount of the adsorbate adsorbed on a solid adsorbent against either the relative pressure or the equilibrium pressure of the adsorbate at a constant temperature. In order to utilize the BET equation accurately to determine surface area, one must at least obtain a sufficient number of data points on the adsorption isotherm to be able to determine the point on the adsorption isotherm at which the "monolayer capacity" occurs. The "monolayer capacity" is a variable in the BET equation and represents the point on the adsorption isotherm, wherein a monolayer of closely packed adsorbed molecules is present at the surface of the adsorbent. Since the monolayer capacity generally occurred at prior to reaching an adsorptive relative pressure of 0.35, one desires to know the adsorption isotherm at least up to this value of relative pressure to be able to calculate the surface area from the BET equation. The adsorptive relative pressure is one way of expressing the equilibrium pressure of the adsorptive as a fraction of the pressure at which bulk condensation of the adsorptive occurs under any set of constant volume and temperature conditions. A concise review of the BET method appears in a publication by the British Standard Institution, BS 4359: Part 1: 1984, titled, "Determination of the Specific Surface Area of Powders-Part I Recommendations for Gas Adsorption (BET) Methods."
Adsorption isotherms can be determined by measuring the sample pressure and determining the amount of adsorbate adsorbed either with a volumetric or a gravimetric method. Use of this invention is applicable to both volumetric and gravimetric determinations of isotherms, although volumetric methods are preferred.
Three main volumetric techniques are in common use. These can be classified as static or fully equilibrated, continuous flow or quasi-equilibrated, and dynamic or chromatographic. Both the static and continuous flow techniques can be described as vacuum volumetric methods. However, in some publications, the dynamic technique has been described as continuous flow, although it does not use vacuum technology, but instead employs a non-adsorbing carrier gas and adsorptive mixture. For the purposes of this invention, the term "dynamic" is applied to all of the chromatographic types of sorption methods commonly used for rapid quality control analysis. As related to this invention, the static and continuous flow techniques are particularly applicable.
Volumetric methods conventionally employ a selected adsorptive at the most convenient temperature for adsorption. For example, when one uses the adsorptive nitrogen with an adsorbent sample to be tested, the adsorptive is cooled to a temperature of approximately 77K. The temperature of the adsorptive is provided by means of a liquid nitrogen bath in a dewar which is open to the atmosphere, and therefore the adsorptive has a boiling point equal to the environmental atmospheric pressure.
By definition, the boiling point of a liquid is the temperature of the liquid at which its vapor pressure is equal to atmospheric pressure. When using liquid nitrogen, the boiling point is at a temperature at which the vapor pressure of the liquid nitrogen is equal to or slightly greater than the atmospheric pressure of the environment.
Consequently, since both the adsorbent and the adsorptive are cooled with the liquid nitrogen bath which is open to the atmosphere, the adsorptive gas and the adsorbent sample have a temperature equal to the boiling point of the liquid nitrogen bath. Because of variations in atmospheric pressure which affect the open dewar, or impurities in the liquid nitrogen bath which affect the saturation vapor pressure of the liquid nitrogen, the normal boiling point temperature of the liquid nitrogen changes very slightly from those reported at exactly one atmosphere pressure.
The reason that the adsorptive is cooled to the boiling point of liquid nitrogen is because it is recognized by those skilled in the art that the quantity of physically adsorbed gas at a given relative pressure (a fraction of saturation pressure) increases with decreasing temperature. Consequently, consistent with practical limits, the lowest most conveniently achieved temperature is chosen to provide maximum measurement sensitivity.
Volumetric devices typically consist of a gas storage unit and a vacuum source unit connected in parallel to a volumetric measuring device of known Volume V.sub.1, referred to as the doser unit or the manifold unit. The doser unit can be connected alternately to either the vacuum unit or the gas storage unit by a series of valves. The doser unit in turn is connected in series through another valve to a sample unit, a chamber of known Volume V.sub.2, which holds the solid sample to be tested. By manipulating the appropriate valves, the doser and sample units are evacuated and the evacuated doser is sealed off from the evacuated sample chamber. Nitrogen is permitted to slowly enter and fill the doser unit from the gas storage unit to a targeted pressure, at which time the valves are closed to seal off the doser, and the nitrogen pressure therein is measured. When a constant pressure P.sub.1 in the doser is achieved, the valve separating the sample chamber and doser is opened allowing the adsorptive, typically N.sub.2, in the doser to expand into the sample chamber.
The sample chamber and doser together define a third Volume V.sub.3 (i.e. V.sub.1 +V.sub.2). When the pressure in V.sub.3 is constant, indicative of adsorption equilibrium, it is measured. This equilibrium pressure is used to calculate the total number of moles of N.sub.2 that remains in the gas phase. The number of moles of N.sub.2 adsorbed on the solid is equal to the number of moles of N.sub.2 initially present in Volume V.sub.1 of the doser, plus the number of moles of N.sub.2 in the sample chamber defining Volume V.sub.2 (the number of moles in Volume V.sub.1 for the initial run is 0, but increases with each successive run), less the number of moles of gaseous N.sub.2 in Volume V.sub.3, after equilibrium. The combined data of the amount of N.sub.2 adsorbed at a particular equilibrium pressure constitutes a single point on the adsorption isotherm. The above procedure is repeated to obtain additional points on the adsorption isotherm. Each successive dose increases the pressure in the sample chamber until, at approximately atmospheric pressure, the sample becomes completely saturated with condensed N.sub.2. At this saturation point, bulk condensation of N.sub.2 takes place around the sample and in the free space in the sample holder. Conventional practice is to generate about 3 to 10 data points on the adsorption isotherm for surface area determinations. A detailed summary of this method is provided in the review paper, "The BET Method of Analysis of Gas Adsorption Data and Its Relevance to the Calculation of Surface Areas" by Dollimore, D., Sponner, P., and Turner, A., Surface Technology, Vol. 4, p. 121-160 (1976).
However, with manual dosing methods, exact target pressures are rarely achieved and several additional unwanted data points can be obtained. While the adsorptive gas is being dosed to the manifold in order to reach an estimated required pressure, an increase above the pressure desired can be obtained. If this higher than required pressure is dosed to the sample, significant loss of operational range can result if too much gas is adsorbed. It is normal practice to open the nitrogen gas adsorptive valve very carefully. This valve typically is capable of supplying nitrogen to higher than atmospheric pressure. If an increase above the pressure desired does occur, then it must be removed by judicious opening of the vacuum valve, before adsorption. Additional pressure stabilizing time is always required and typically is equal to one or two minutes. This time is in addition to the sorption equilibration time.
A capillary method having a continuous flow of the adsorptive is disclosed in U.S. Pat. No. 2,729,969 to Innes. Innes teaches a method for the measurement of surface areas greater than 0.5 meter per gram, which comprises introducing nitrogen gas at a constant flow rate into an evacuated system containing a weighed sample amount which is cooled to -195.degree. C., measuring the time required for the vacuum in the evacuated system to decrease from 29.6" to 23.7" of mercury and calculating from the time required the surface area of the material. Innes further teaches that due to impurities present in the liquid nitrogen cooling bath, the bath temperature is somewhat higher than the boiling point of pure nitrogen. As a result, the saturation vapor pressure is slightly above one atmosphere. It has been reported elsewhere that dissolved impurities usually increase the bath temperature sufficiently to cause the vapor pressure of pure liquid nitrogen in the sample cell to increase by 10 to 20 mm Hg above ambient pressure. Innes further discloses that other gases such as n-butane, argon, CO.sub.2, CO, and gases having a vapor pressure of about one atmosphere at 50 to 225K can be employed for area and pore volume measurements. However, the Innes method suffers from environmental induced flow rate fluctuations and imprecise equilibrium pressure conditions, which affect the accuracy of measurement.
The just-mentioned and other similar problems have been addressed in U.S. Pat. No. 4,489,593 to Pieters, et al.
Innes and Pieters, et al. disclose using an adsorptive at a temperature at which the adsorptive condenses at approximately one atmosphere pressure and introducing the adsorptive into the sample holder containing the substance to be analyzed at a constant flow rate. Pieters et al. teach controlling the flow rate by the use of a mass flow controller. By using such techniques, the flow rate approximates the adsorptive equilibration rate of adsorption. By controlling the mass flow rate to be not greater than the equilibration rate of adsorption, the pressure established, at any given time during the introduction of the adsorptive, will be the equilibrium pressure. This is significant because the adsorption isotherm is a plot of the amount of adsorptive adsorbed at a given equilibrium pressure. Consequently the determination of the adsorption isotherm is simplified. However, owing to the flow rate being not greater than the equilibration rate of adsorption, flow times of about four hours are required. Moreover, it is generally recognized that the equilibration rate of adsorption can vary according to the physical and chemical characteristics of the sample.
Another technique for determining adsorption isotherms has been reported in Nelsen & Eggersten, Analytical Chem., Vol. 30, p. 13-87 (1958), "Adsorption Measurements By A Continuous Flow Method." In this dynamic technique, nitrogen is adsorbed by the adsorbent at a liquid nitrogen temperature from a gas stream of nitrogen and helium, and eluted upon warming the sample. The nitrogen liberated is measured by a thermal conductivity detector. Thus, the amount of adsorbed gas is determined by concentration measurements in a continuous flow system at atmospheric pressure, rather than by pressure volume measurements at below atmospheric pressure. This method is referred to herein as a chromatographic method for determining adsorption isotherms because of its resemblance to chromatography techniques. Two requirements of this method are steady flow of carrier and adsorbate gases, and thorough mixing of the two gases, in situ.
A typical continuous flow process (dynamic technique) is defined in U.S. Pat. No. 3,211,006 to A. J. Haley, Jr. In Haley, Jr., a thermal conductivity cell is employed as a detector for the amount of adsorption from an operating gas mixture, which is a maximum of approximately fifty percent (50%) nitrogen at operating pressure of slightly over atmospheric to about 200 atmospheres. However, this continuous method requires several different inert carrier gas/adsorbate gas mixtures to determine the isotherm. As such, the gas mixtures typically are expensive to purchase pre-mixed. Alternatively, it is time-consuming to obtain the gas mixture by mixing gases which would require two mass flow controllers, each of which will contribute to inaccuracy of measurement.
In a similar dynamic method (continuous flow), U.S. Pat. No. 4,856,320 to Bose et al. employs operating pressures from approximately 0.5 to 116 atmospheres, an operating temperature of approximately 25.degree. C. and measuring the dielectric constant of the gas to determine the amount of gas adsorbed. Bose et al. teach that the volumetric method is reliable at low pressure when all gases in the bulk phase closely resemble the ideal gas. However, the Bose et al. method lacks precision resulting from small adsorption volumes and use of polar gases or very high pressures which affects the second dielectric virial coefficient.
Desorption isotherms are important because various mathematical equations have been developed to enable one skilled in the art to determine certain morphological characteristics of solids. More specifically, adsorption, as well as desorption isotherms enable one to calculate the pore size distribution of a solid sample from the data embodied therein. A desorption isotherm is a plot of the amount of a preadsorbed gaseous material, the desorbate, desorbed from a solid against the equilibrium pressure or relative pressure of the desorbate at a constant temperature. After desorption from a solid sample, the desorbent, the gas is referred to as the desorptive. The desorption isotherm differs from the adsorption isotherm in that it is constructed starting with a solid, saturated with the desorbate, and gradually reducing the pressure over the solid to near absolute vacuum. In contrast, the adsorption isotherm starts with an evacuated solid sample and increases the pressure of a gaseous adsorptive in contact therewith until sample saturation is reached. The adsorption and desorption isotherms are collectively known as the sorption isotherm.
Gas-solid interaction can cause at least a portion of the desorption path of the sorption isotherm to lie higher on the isotherm plot than the adsorption path. The failure of the desorption path to duplicate the adsorption path of the isotherm is referred to as hysteresis. The two most common forms of hysteresis are closed loop and open loop. In the closed loop hysteresis behavior, the desorption path of the isotherm eventually rejoins the adsorption path at some low relative pressure. Closed loop hysteresis normally is associated with porosity in the sample being tested.
For example, at the start of the desorption isotherm, the pores of the sample are saturated and filled with the desorbate. As desorption occurs, capillary action delays desorption of the desorbate present within the pores, such that a lower pressure is required to evacuate the pores relative to the pressure which initiated the filling of the pores during adsorption. This delay is expressed as closed loop hysteresis behavior of the sorption isotherm. Open loop hysteresis is characterized by the failure of the desorption path of the isotherm to rejoin with the adsorption path. Open loop hysteresis usually is associated with some measurable amount of irreversible adsorption, which typically occurs when the gas reacts with the solid sample upon adsorption, conventionally referred to as chemisorption. Consequently on desorption, less material will desorb than was initially adsorbed, giving rise to an open loop in the sorption isotherm.
By intentionally inducing chemisorption, much can be learned about the surface of the solid sample. For example, chemisorption can be employed to determine the percent dispersion and surface area of microscopic particles of a catalyst deposited on a support by employing a gaseous adsorbate which will undergo chemisorption with the catalyst particles but not the support.
Other information in the substantially complete sorption isotherm permits the determination of total pore volume, average pore size, and pore shape, for example, slits versus cylindrical pores.
The above discussion highlights only a few of the incentives for obtaining substantially complete pictures of the entire sorption isotherm rather than narrow segments thereof, and any method or device capable of producing substantially complete sorption isotherms quickly and accurately possesses substantial advantages.
In view of the above, it is evident that there has been a continuing search for a quicker, simpler, and more accurate methods for determining surface area and sorption isotherms. The present invention was developed in response to this search.